Is there a way that you wouldn't have to use ballast, or place the battery up front where the ballast would go.
You don't want weight on fast robots, (but maybe you do) any hints on what your robot is, or what it looks like.

You can buy 5 and 10 lb iron weights at discount department stores, and the have a convenient hole thru the center for mounting.

And Small Parts still sells 27 pound bricks of lead. We have used them in the past as ballast (many, many years ago - ahhhh, for the good old days, when our robots always seemed to come in well under weight instead of well over... )

-dave

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__________________ "I know what you're thinking, punk," hissed Wordy Harry to his new editor, "you're thinking, 'Did he use six superfluous adjectives or only five?' - and to tell the truth, I forgot myself in all this excitement; but being as this is English, the most powerful language in the world, whose subtle nuances will blow your head clean off, you've got to ask yourself one question: 'Do I feel loquacious?' - well do you, punk?"
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And Small Parts still sells 27 pound bricks of lead. We have used them in the past as ballast (many, many years ago - ahhhh, for the good old days, when our robots always seemed to come in well under weight instead of well over... )

-dave

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Hmmm. Years of handling lead; that might explain some things...

On a philosophical note, does a battery count as a battery if it's not part of the electrical system? Yes, yes, I know the rules about one and only one 12v primary battery, but if it's not plugged into anything is it really a battery? Is it not just a battery shaped weight? Not that I'm advocating for using extra batteries as weights, but still an interesting point of discussion.

On a philosophical note, does a battery count as a battery if it's not part of the electrical system? Yes, yes, I know the rules about one and only one 12v primary battery, but if it's not plugged into anything is it really a battery? Is it not just a battery shaped weight? Not that I'm advocating for using extra batteries as weights, but still an interesting point of discussion.

Is a battery a battery if it's not part of an electrical system? If not, then someone needs to go after Duracell and Energizer for false advertising.[/sarcasm]

Seriously, I would argue that a battery is a battery at any time. If it is only a battery when current is flowing, then anytime the main breaker is hit, the battery becomes a battery-shaped weight. If it is only a battery when hooked into the electrical system, then you're charging a battery-shaped weight. (Don't get any ideas.)

More importantly, a battery has the potential to suddenly become a spark generator at any time. As soon as it hits metal on both contacts, it becomes a safety hazard, but it may or may not be plugged into an electrical system. Is it not a battery then?

Gentlemen,
Although lead seems to be an ideal ballast material, remember that in some areas lead is considered a hazardous material. It may not be cut or machined at events and should be painted or coated to prevent students (and mentors) from ingesting contaminants while eating with their hands.

More importantly, a battery has the potential to suddenly become a spark generator at any time. As soon as it hits metal on both contacts, it becomes a safety hazard, but it may or may not be plugged into an electrical system. Is it not a battery then?

Ahh, so a battery by any other name is still a battery. I think this is the most important argument against using a battery as a weight, especially when there is plenty of other heavy stuff around that can be used. How about a box of old motors?